QUANTITATIVE BREMSSTRAHLUNG SINGLE-PHOTON EMISSION COMPUTED TOMOGRAPHIC IMAGING - USE FOR VOLUME, ACTIVITY, AND ABSORBED DOSE CALCULATIONS

Purpose: To perform bremsstrahlung single photon emission computed tom ographic (SPECT) imaging using P-32 chromic phosphate for volume and a ctivity quantitation to calculate absorbed dose estimates, Methods and Materials: Seven cancer patients enrolled in clinical Phase I therape utic protocols were injected with 2.5 million particles of macroaggreg ated albumin, followed by colloidal P-32 chromic phosphate by direct i nterstitial injection into the tumor-bearing region under computed tom ographic (CT) guidance, SPECT images were obtained in these patients, The patient body contour was defined through the use of two externally placed Compton backscatter Tc-99m sources, A computer algorithm was w ritten to facilitate region-of-interest volume and activity determinat ion on the reconstructed SPECT slices based on a fixed threshold metho d, Three sequential SPECT studies were acquired in two of these patien ts, to determine the accuracy of activity quantitation for bremsstrahl ung SPECT studies using Chang's postprocessing method of attenuation c ompensation with a computer-generated body contour based on the Compto n backscatter sources, and an experimentally measured effective linear attenuation coefficient for P-32, The serial data in these two patien ts were used to calculate absorbed dose estimates, Results: The Tc-99m backscatter sources enabled the patient body outline to be clearly vi sualized in all the transaxial reconstructed slices and did not contri bute significant counts to the patient P-32 counts, The calculated act ivities from the SPECT studies were within 7.8% of the administered P- 32 activity, The two calculated patient absorbed doses were 4.2 X 10(3 ) Gy and 5.9 x 10(3) Gy for injected activities of 736 MBq and 920 MBq , respectively, Conclusion: We conclude that accurate quantitative bre msstrahlung SPECT imaging, for the case of high contrast well-localize d activity distributions, with a commercially available postprocessing attenuation correction algorithm, can be performed in a clinical sett ing, Entirely SPECT-based measurements can be used to generate absorbe d dose estimates.